The inventions relates to a process and a device for preparing homopolymers and copolymers, including those of styrene, butadiene and (meth)acrylic acid and derivatives thereof, by the emulsion polymerization technique.
The manner in which polymerizations are carried out involves bringing one or more polymnerizable monomers into disperse distribution in a liquid which is ideally inert in the reactionxe2x80x94usually waterxe2x80x94in presence of detergents or soaps as dispersing auxiliaries. Polymerization takes place predominantly by means of initiator radicals in the monomer-containing micelles that are formed. High molecule masses can be obtained in such a polymerization, since monomer is able to make its way continually into the micelles. The mechanism is normally that of a free-radical polymerization; the reaction products can in many cases be processed further directly in dispersion form (as, for example, in the case of the production of paints and adhesives. Known products are homo- and copolymers (P) of styrere (S), vinyl chloride (VC) butadiene (Bu) or methyl methacrylate (MMA). The particle size and its distribution can often be controlled by the use of see (particles), added to start with or produced in situ. Typical conditions in an industrial-scale process (taking the example of a styrene-butadiene copolymer or a polyacrylate) are reaction periods of from 3 to 12 h at from 40 to 100xc2x0 C.
DE-A 23 32 637describes an emulsion polymerization in which butadiene is reacted with comonomers such as styrene, acrylonitrile (AN) or esters of acrylic or methacrylic acid in the presence of customary emulsifying auxiliaries, such as higher fatty acids, higher alkyl (aryl)sulfonates, adducts of alkylene oxides with long-chain fatty alcohols, and free-radical initiators such as alkali metal persulfates, at more than 115xc2x0 C. An advantage over the prior art with operation at less than 80xc2x0 C. is said to be the higher polymerization rate. However, there is often an adverse effect on the performance properties of products prepared at such high temperatures, in respect for example of the molecular mass distribution, the particle size distribution and, in association therewith, the adhesive strength, for example. In addition, safety aspects (such as the pressure developed when butadiene is a component) are increasingly playing an important role in connection with the reaction regime.
Nevertheless, any reaction regime at higher temperaturesxe2x80x94in other words, more than 80xc2x0 C., in particular more than 85xc2x0 C.xe2x80x94is an important parameter for large-scale industrial plant, since the reaction times can be significantly reduced; in other words, a large-scale industrial process is carried out with lower cycle times, which helps save on investment costs for a greater number of plant units. A major problem area which then requires solution, however, is the dissipation of heat, for example in order to help avoid instances of local overheating in the case of exothermic reactions, since such overheating can in many cases lead to secondary reactions, irregular molecular mass distributions or irregular particle sizes. EP-A 0 486 262discloses the preparation of emulsion copolymers where the energy balance is monitored and the result is used to control the supply of the comonomers and the temperature. Temperature control is effected by the use, inter alia, of an external heat exchanger. No information is given about the quality of the products or the design of the pumps or heat exchanger.
A heat treatment of homo- or copolymers of VC, by means for example of an external heat exchanger, for reducing viscosity after the actual emulsion polymerization is described in Research Disclosure July 1978, reference 17149, p. 17. No mention is made of influencing the actual main reaction.
EP-A 0 608 567, for use in the suspension polymerization of VC to form homo- or copolymers, in a vessel with stirrer and an external heat exchanger, describes a special pump (Hydrostal pump) by means of which the reaction mixture is guided at an angle of 90xc2x0C., the interior having a conical hub with a rotor blade which movies with a spirally rotating motion. No remarks are made about the heat exchanger. Stirring energy and circulation energy must he kept within a certain proportion. A comparable pump is used in EP-B 0 526 741 as well, which also deals with the suspension polymerization of VC; there, the type of heat exchanger is regarded as not being critical (see p. 4, lines 36 to 40).
In the process for preparing emulsion polymers of DE-A 44 42 577, the energy liberated in the course of the exothermic reaction is dissipated in part by distilling off a water/monomer mixture under reduced pressure from the reaction vessel (a stirred reactor). Although this measure does lead to a certain reduction in the polymerization period, i.e. essentially in the time required for adding the monomer or monomers, it is still not sufficiently suitable for large-scale industrial plant, especially since there is little or no provision for it to be used widely, for example for low-boiling comonomers and (co)monomers which are gaseous under standard conditions (for example, comonomers of the butadiene type).
It is an object of the present invention to find a preparation process which can be carried out on an industrial scale and has a broad field of use but which does not have the disadvantages of the prior art. In other words, in particular, short reaction times should be possible, a broad spectrum of different monomers, including those which are gaseous under standard conditions should be accepted, and the products should be at least comparable in terms of the performance properties with current products.
We have found that this object is achieved by a process for preparing homo- or copolymers of at least one of the polymerizable monomers of is the group consisting of styrene, butadiene, vinyl chloride, vinyl acetate, vinylidene chloride, alkyl (meth)acrylate (meth)acrylic acid, (meth)acrylonitrile and (meth)acrylamide in an emulsion polymerization technique at at lease 40xc2x0 C. in the presence of a dispersing auxiliary and of a free-radical polymerization initiator. The novel process comprises preparing the polymer, at least 85% by weight of which is formed from one or more of these monomers, in the following stages, where
a) in a first stage water is added as a solvent which is inert in the reaction, and dispersing auxiliaries, seed and a first portion of monomer(s) arc added if desired,
b) in a second stage initiator is added, and
c) in a third stage the remainder or all of the monomer(s) is added directly or in emulsion form and in the presence of further water and, if desired, further dispersing auxiliary or other auxiliaries, it also being possible to operate the stage a) and b) or b) and c) in each case as single stages, and in certain stages or every stage moving the reaction mixture in its dispersion form by means of an external circuit which leads from and back to the reaction vessel and comprises at at least one low-shear pump and at least one heat exchanger having an essentially laminar flow profile, and carrying out polymerization at from 40 to 120xc2x0 C.
In preferred embodiments of the novel process at least 90% by weight, in particular at least 93% by weight, of the polymer consists of one or more of the abovementioned monomers, and the polymerization is carried out at from 50 to 100xc2x0C., in particular from 60 to 95xc2x0 C. and, very particularly, from 70 to 95xc2x0C. Of the monomers mentioned, preference is given to styrene, butadiene, alkyl (meth)acrylate and (meth)acrylonitrile.
The novel process can be carried out either by including a portion of monomer(s) in the initial charge in stage a) or in a combination of a) and b) and subsequently, in stage c) or in a combination of b) and c), adding the remainder, or by supplying the total amount exclusively in stage c) or in a combination of b) and c). If monomer(s) is (are) included in the initial charge in stage a) or in a combination of a) and b), then the amount thereof is judiciously from 3 to 30% by weight of the total amount of monomer(s) to be supplied, preferably from 5 to 25% by weight and, with particular preference, from 8 to 20% by weight. Monomers of low or zero solubility in water are judiciously supplied in emulsion form, i.e with water and dispersing auxiliary.
Other suitable comonomers in addition to the monomers mentioned above include vinyltoluene, N-methylol(meth)acrylamide and C2-C3-hydroxyallyl (meth)acrylate. Unless specified otherwise, alkyl in the monomer names means a C1-C9 linear or C3-C9 branched radical, especially methyl, ethyl, isopropyl, n-butyl, tert-butyl, isobutyl or 2-ethylhexyl.
The dispersing auxiliaries can be included in the initial charge prior to the addition of the monomer or monomers or can be supplied in addition to the monomers or in the monomer emulsion The amount added, relative to the overall amount of monomer(s) as 100% by weight, is from 0.01 to 10% by weight, preferably from 0.05 to 8% by weight (and also depends on the type: whether nonionic or anionic). Known compounds arexe2x80x94in addition to natural soapsxe2x80x94alkyl polyglycol ethers, such as ethoxylated lauryl alcohol, alkylpbenol polyglorcol ethers, such as those of nonylphenols or salts of long-chain alkyl, aryl or alkyaryl acids, such as Na-lauryl sulfate. In addition to these it is also possible for protective colloids to be present (in amounts of from 0.001 to 15% by weight), such as cellulose ethers or polyvinyl alcohol. In a preferred embodiment of the invention dispersing auxiliaries are present in stage a) either together with seed or with the first portion of monomer(s); it is also possible that all three components are already present in stage a). Latest in stage c) dispersing auxiliaries have to be present.
The seed is either produced in situ or included in the initial charge; if desired, it can he added at various points in time, in order, for example, to bring about a polydisperse or polymodal (e.g. bimodal) distribution. The proportions relative to the amount of monomer(s) as 100% by weight are frequently from 0.1 to 5.0% by weight, preferably from 0.2 to 3.0% by weight. In a preferred embodiment of the invention seed is present in stage a) either together with the first portion of monomer(s) or not.
Suitable polymerization initiators for the beginning, for the complete implementation and/or for the continuation of the reaction are water insoluble or, preferably, water-soluble compounds. Examples of the known free-radical initiators include hydrogen peroxide, peroxodisulfuric acid and its salts, for example K or NH4 peroxodizulfate, dibeizoyl peroxide, lauryl peroxide, tri-tert-butyl peroxide, azodiisobutyronitrile, alone or together with reducing components such as Na bisulfite, ascorbic acid or Fe(II) salts, for example tert-butyl hydropeioxide with Na bisulfite-formaldehyde adduct or Na bisulfite-acetone adduct. The polymerization initiator can be added in one or more stages, and in the latter case in varying amounts and by varying methods; for example, in the case of more complex systems, such as redox initiators, it is also possible for some to be included in the initial charge aid the remainder added continuously thereafter. In many cases the polymerization initiator is metered in in parallel with the monomer; it can be added in a time which is shorter or else longer than that for addition of the monomer.
In the course of the reaction it is also possible to add up to 5% by weightxe2x80x94based on the proportion of monomer(s) as 100% by weight of auxiliaries such as molecular weight regulators, further surfactants, acids. salts or complexing agents. In case the polymerization is carried out in presence of 3 to 10% by weight of a liquid organic expanding agent, also expandable (expanded respectively) polymers, e.g. expandable polystyrene can be produced.
For the purpose of formulation, for example in order to increase the storage stability, the end product of the emulsion polymerization can have added to it alkalis such as aqueous NaOH solution or bases (such as NH3 or appropriate amines) in order to establish a pH of from 4 to 10. further known additives are preservatives such as microbicides, film formers or leveling agents, antifoams or resin emulsions which increase the adhesion (tackifiers).
The equipment employed in the external circuit is especially suitable for a large-scale industrial regime.
The low-shear pump (or pumps) must have a low shear effect on the emulsion must withstand pressures of, for example, up to 15 bar, must be insensitive to gases in the emulsion, must allow a good hourly throughput of up to 100 m3/h, preferably up to 60 M3/h, particularly preferably up to 45 m3/h, and must also be resistant at more than 100xc2x0 C. and easy to clean. Customary rotary piston pumps or gear pumps are unsuitable for the novel process. Particularly suitable pumps are nonclogging pumps which operate in accordance with the vortex principle; also possible are displacement pumps, monopumps or disk flow pumps and any pumps of a type which ensures a minimum of shear forces in order to give little or no disruption to the relatively unstable state of both the reaction mixture and the finished product emulsion. The pumps can preferably be sealed with double floating-ring seals in a back-to-back arrangement.
The heat exchanger or exchangers has or have a substantially laminiar flow profile; in other words, the action of shear forces should be minimal and, where possible, no dead zones (that is, zones not traversed by the flow) should occur. Known heat exchangers of the plate type tend to be unsuitable, since the narrow gaps and deflections mean that the mechanical resistances offered are too great; moreover, they are less suitable for a highly pressure-resistant configuration and are more difficult to clean.
The novel process is particularly suitable for preparing aqueous polymer dispersions whose films have a low glass transition temperature (DSC method); it is particularly suitable in connection with glass transition temperatures of less than 150xc2x0 C. preferably less than 100xc2x0 C., in particular less than 50xc2x0 C. Furthermore, it also proves to be suitable for polymer dispersions having a mean particle size, of, preferably, from 50 to 2000 mn, in particular from 100 to 1500 nm. The polymer dispersion has a viscosity, in particular, of from 30 to 1500 mPas; in the course of polymerization, the viscosity may also be higher or lower.